This invention relates to an object movement measuring apparatus.
The present applicant has proposed a three-dimensional object measuring apparatus in which a streak camera is used to detect the surface configuration and the internal structure of a three-dimensional object (cf. Japanese Patent Application Laid-open No. 277538/1987 (Dec. 2, 1987) and U.S. Pat. application Ser. No. 074,879 (July 17, 1987)).
One example of the three-dimensional object measuring apparatus, as shown in FIG. 5, comprises: a pulse laser 1, beam splitters 2 and 4; a beam expander 3; a total reflection mirror 5; a stationary reflection mirror 6; a movable reflection mirror 7; shutter apertures 8 and 9; total reflection mirrors 10 and 11; an image forming lens 13; an input slit 14; a streak camera 15; a detector 16; an amplifier 17; a delay circuit 18; a reading device 19; and a monitor 20. In FIG. 5, reference numeral 12 designates an object under measurement.
A pulse beam from the pulse laser 1 is split by the beam splitter 2 into two parts, one of which is applied to the detector 16, where it is converted into an electrical signal. The electrical signal is applied, as a gate trigger signal, to the streak camera through the delay circuit 18. On the other hand, the beam passed through the beam splitter 2 is expanded by the beam expander 3 and split by the beam splitter 4. The beam passed through the beam splitter 4 is applied to the object 12 from above with the aid of the total reflection mirrors 5 and 10. At the same time, the beam reflected by the beam splitter 4 is reflected by the stationary reflection mirror 6 and the movable reflection mirror 7 which form an optical delay system and by the total reflection mirror 11 so that it is applied to the object 12 from below. That is, the object is irradiated from above and below in the above-described manner. The light beams reflected from the surface of the object 12 go backward along the incident optical paths, while the light beams passed through the object 12 advance along the optical paths which are opposite to the incident optical paths, respectively, so that they are applied through the image forming lens 13 to the streak camera 15. In this case, according to the purpose of the measurement, the lens 13 is adjusted so that the light beam from the front or back surface of the object is focused, or instead of the lens 13 a zoom lens is used so that a part of the object can be observed with different magnifications. The streak image formed by the streak camera is read with the reading device 19, and is observed and subjected to some operation such ad analyzing.
In the apparatus described above, a light beam reflected from an object 12 having a recess as shown in FIG. 6 is applied to the streak camera with a time delay corresponding to two times the depth of the recess. Therefore, the depth d.sub.1 of the recess is: EQU d.sub.1 =c.multidot.t/2
where c is the velocity of light, and t is the delay time of the streak image corresponding to the recess of d.sub.1. Thus, the outside dimension of the object can be obtained. In FIG. 6, .DELTA.t means a width of the pulse beam.
On the other hand, if it is assumed that the object uniformly has a refractive index n in an internal structure measurement, then the transmission time difference t is: EQU t=(n/c)(W.sub.2 -W.sub.1).
Therefore, by measuring t from the streak image, the value n can be obtained when (W.sub.2 -W.sub.1) is known.
FIG. 7 shows one example of a conventional object internal structure measuring apparatus. In FIG. 7, those components which have been described with reference to FIG. 5 are therefore designated by the same reference numerals. In FIG. 7, reference numeral 21 designates an optical fiber; and 22, an image forming optical system including a lens 23, a half-mirror 24 and a total reflection mirror 25.
In the apparatus of FIG. 7, the pulse beam from the pulse laser 1 is applied through the optical fiber 21 to the inside of the object 12 under measurement and radiated at a suitable solid angle, and the beam passed through the object by the radiation is focused on the photocathode of the streak tube in the streak camera 15 by means of the image-forming optical system 22. As was described above, the image-forming optical system 22 is made up of the lens 23, the half-mirror 24 and the total reflection mirror 25. That is, the system 22 is so designed that the image formed through the half-mirror 24 and the image formed through the total reflection mirror 25 are subjected to analysis.
The pulse beam emerging from the end of the inside part of the object is a spherical wave. Therefore, the streak image formed by the streak camera is such that the image of the internal structure of the object is superposed on the spherical-wave-shaped pulse beam image. Accordingly, the internal structure of the object can be detected by the following method: with the analyzing device (the monitor) 20, the curvature of the spherical wave of the light beam emerging from the optical fiber is calculated in advance, and the curvature thus calculated is removed from the streak image.
On the other hand, in measuring an object in motion with the above-described three-dimensional object measuring apparatus, all the images of the objects in motion is integrated as the output image of the streak camera, and therefore it is impossible to evaluate the motion of the object.
Heretofore, an apparatus utilizing moire fringes or slit pattern projection is used to optically monitor the movement of an object in a sectional direction.
FIG. 8 is a diagram for a description of the formation of moire fringes. FIG. 9 is a diagram showing one example of a moire-fringes-reading optical system. In FIG. 9, reference characters G.sub.1 and G.sub.2 designate gratings; L.sub.1 and L.sub.2, lenses; Q, a lamp as a light source; P, a photoelectric conversion device; and S, an aperture (lens stop).
When the two gratings G.sub.1 and G.sub.2 which are equal in grating interval are piled one on another in such a manner that they form a small angle .theta., moire fringes are formed, as shown in FIG. 8. When, with the grating G.sub.2 fixed, the grating G.sub.1 is moved by a grating interval d in a direction perpendicular to the direction of grating, then the moire fringe pattern is moved by one fringe. This distance of movement l can be calculated as follows: EQU l=d/.theta..
Therefore, the movement of the grating G.sub.1 can be determined by detecting the movement of the fringes.
FIG. 10 shows one example of the projection of a slit pattern on an object under measurement.
As shown in FIG. 10, a parallel straight line grating (opaque) having a pitch s is placed over an object, and the object is irradiated through the grating by a point light source LS at a distance L from the grating so that it is observed with the eye at the same distance L from the grating. In this case, what are observed bright are the parts of the object's surface which are irradiated by the light source and can be seen with the eye (intersections of solid lines and dashed lines). Such parts (bright points) appear in the form of layers arranged in the direction of depth. In FIG. 10, "N (the order of a bright point layer)=i" means the fact that in the i-th layer below the grating the parts irradiated by the light source coincide with those which can be seen with the eye. The distance h.sub.N of an N-th bright point layer from the grating can be obtained as follows: The bright points of the N-th layer are defined by the intersections of the solid lines and the broken lines which pass through the grating gaps, and therefore ##EQU1## where D is the distance between the light source and the eye measured in parallel with the grating surface.
The above-described equation can be rewritten as follows: EQU h.sub.N =N L s / (D-N s)
Thus, the line connecting the bright points on the N-th (1st, 2nd, . . . . .) layer; that is, the contour line formed by connecting the bright points equal in depth from the grating surface is formed on the object.
In this case also, the movement of the object can be obtained by detecting the movement of the bright points.
However, the above-described method of detecting the movement of an object by utilizing moire fringes is disadvantageous in that an apparatus for practicing the method is unavoidably intricate in construction. That is, in the apparatus, to detect the movement along the direction of movement it is necessary to provide two windows in such a manner that they are disposed in parallel with the moire fringes and spaced by a quarter (1/4) of the pitch l, to convert the luminances of the windows into electrical signals by photoelectric conversion devices, and to determine whether or not one of the electrical signals leads or lags the other in phase. When it is required to detect the movement of the object in an arbitrary direction, it is necessary to provide two more windows in a direction perpendicular to the moire fringes for phase detection. In this case, the apparatus is more intricate in construction.
In the method of projecting a slit pattern on an object, as is apparent from the above description, the motion of the object cannot be measured without trigonometry, and it is difficult to detect the high speed motion of an object at a given time instant and analyze it.